The present invention, in some embodiments thereof, relates to chemistry of metals, and more particularly, but not exclusively, to a method of activating a surface of self-passivated metals, such as aluminum, to a method of reactivating metal oxides and to uses of these methods in, for example, electrochemical cells.
When exposed to air at room temperature, or another oxidizing environment, some metals, such as aluminum, and metal alloys, such as stainless steel, tend to form a hard, relatively inert surface. This phenomenon is known as self-passivation or natural passivation. In the case of aluminum, for example, a surface layer of amorphous aluminum oxide about 2-3 nm thick is formed naturally, providing an effective protective layer against corrosion and other chemical processes in specific environmental conditions. Aluminum alloys typically form a thicker oxide layer, 5-15 nm thick, but tend to be more susceptible to corrosion.
Like aluminum, also silicon, a self-passivating metalloid, is naturally unreactive electrochemically, typically covered with a natural passivating layer of oxides formed in ambient conditions. Hydrofluoric acid (HF) is typically used for silicon surface activation, and is also an industrially utilized agent for silicon oxide (silica, SiO2) dissolution. However, aluminum oxide (alumina, Al2O3) is far less soluble in hydrofluoric acid; 2-3 orders of magnitude lower compared to silica [Kurt R. et al., J. Am. Ceram. Soc., 82 [12] 3561-66 (1999)], making hydrofluoric acid a less effective agent for aluminum activation.
Battery systems based on aluminum and aluminum alloys as anodes present a potential for efficient, inexpensive, and high performance power sources. The main advantages of aluminum-based battery systems include high energy content (8 kWh/kg), low equivalent weight, high natural abundance (low price), and safety characteristics, as well as relatively non-toxic and environmentally safe byproducts. For example, in the context of electric vehicle propulsion, aluminum contains approximately one-half the energy content of gasoline per unit weight and three times the energy per unit volume.
One of the main obstacles still challenging those who attempt to use aluminum as a source of fuel (an anode) in an electrochemical cell based on non-aqueous solutions, is overcoming its tendency to self-passivate by oxides or other protective layers, which causes the metal anode to be less electrochemically reactive and thus unusable as an anode. In alkaline aqueous solutions aluminum (but also any other anode applied) based power sources suffer from a variety of problems. Among those one can include severe anodic weight loss due to corrosion reactions that significantly reduce battery energy capacity and also degradation of the electrolyte itself by formation of insoluble products.
The aluminum air (Al/Air) system theoretically represents a viable metal anode/air (oxygen) cathode battery in terms of energy capacity and cell potential. However, to date, there are no commercial battery products which utilize aluminium anode for the above mentioned reasons, namely corrosion and electrolyte degradation, as well as performances degradation in peak humidity conditions (both high and low), and CO2 poisoning (in the alkaline environment), all of which cause a decrease in the reversible electrode potential, i.e., the cell voltage is considerably lower than the theoretical value [Li, Q. et al., J. Power Sources, 2002, 110, p. 1-10].
An aluminum/oxygen system was first demonstrated in the early 1960s by researchers who found that the addition of zinc oxide or certain organic inhibitors, e.g. alkyldimethylbenzyl-ammonium salts, to the electrolyte, significantly decreased the corrosion of amalgamated aluminum anodes in 10 M sodium or potassium hydroxide solutions [Zaromb, S., J. Electrochem. Soc., 1962, 109, p. 1125-1130; Bockstie, L. et al., J. Electrochem. Soc., 1963, 110, p. 267-271].
The major development effort to date has focused on metal/air cells with two types of electrolytes, i.e. alkaline and saline electrolytes. In thermodynamic terms, an aluminum anode should exhibit a potential of −1.66 V in saline and −2.35 V in alkali electrolyte; however, practical aluminum electrodes operate at a significantly lower potential because (a) aluminum is normally covered by an oxide/hydroxide film which causes a delay in reaching a steady-state voltage due to internal resistance; (b) aluminum undergoes a parasitic corrosion reaction, resulting in less than 100% utilization of the metal and the evolution of hydrogen.
The progressive consumption of hydroxyl ions at the aluminum electrode makes the electrolyte more saturated with aluminate (aluminum salt), which eventually exceeds the super-saturation and forms crystals of aluminum hydroxide that precipitate with the regeneration of hydroxyl ions. In addition to the electrochemical consumption of the anode, aluminum is thermodynamically unstable in an alkaline electrolyte and reacts with the electrolyte to generate hydrogen. This parasitic corrosion reaction, or self-discharge, degrades the Coulombic efficiency of the anode and must be suppressed in order to minimize the capacity loss.
Molten salts constitute non-aqueous media that have been considered as alternative electrolyte in which aluminum does not form the surface oxide film. Since aluminum can be electrodeposited from the non-aqueous media, such electrolytes were considered as suitable for developing rechargeable aluminum batteries. Considerable research has been carried out for developing aluminum secondary batteries since the 1970s, with the earliest attempt to develop the Al/Cl2 battery system by using NaCl—(KCl)—AlCl3 as electrolyte [Holleck, G. L. et al., J. Electrochem. Soc., 1972, 119, p. 1161-1166]. Due to the difficulty associated with the chlorine storage, metal chlorides were proposed as the cathode materials; however, the high solubility of metal chlorides in the melts limited the development of such battery systems [Weaving, J. S. et al., J. Power Sources, 1991, 36, p. 537-546]. Sulfur and its group elements were also suggested as the cathode candidates, as well as transition metals and their sulfides, yet for cost and other technical aspects and consideration, FeS2 and FeS are the most commonly used cathodes for the Al based system to date [Li, Q. et al., J. Power Sources, 2002, 110, p. 1-10]. Overall these molten salt battery systems operate at high temperatures (at least over 100° C.), and exhibit high discharging capacity, though a significant capacity loss is observed due to the solubility of metal sulfides, and the formation of aluminum dendrites during the charging still harms the battery efficiency.
Room temperature ionic liquids (RTILs) are a class of solvents, which encompasses a wide range of liquid materials produced from the conjugation of relatively large molecular organic cations (e.g. immadazolium, tetraalkylammonium, sulfonium, piperidinium, pyridinium and betaine), with relatively small inorganic anions (e.g. PF6−, BF4−, AlCl4−, (CF3SO2)2N−, Et3OSO3−). These materials, which are composed only of ions, may be compared to high temperature molten salts, with the obvious difference that the melting point of the RTIL is near room temperature (about 25 C.°). Hagiwara, R. et el. [J. Fluor. Chem., 1999, 99(1) and J. Electrochem. Soc., 2002, 149, D1] reported the synthesis and properties, such as electric conductivity and the thermal stability at elevated temperatures, of fluorohydrogenate-containing RTILs, such as the fluoroanion-RTIL EMIm(HF)2.3F.
Several researchers have explored the use of chloroaluminate RTILs as electrolytes for aluminum batteries [Egan, D. et al., J. Power Sources, 2013, 236, p. 293-310]. These studies were limited by the laborious preparation due to the highly exothermic reaction between 1-ethyl-3-methylimidazolium chloride (EMIC), and AlCl3. Air and water stable ionic liquids such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]TFSI), 1-butyl-1-met hylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP]TFSI), [(Trihexl-tetradecyl)phosphonium] bis(trifluoromethylsulfonyl)imide (P14,6,6,6TFSI) and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm]TFB, were considered as alternative to the chloroaluminate ionic liquids; however, efforts were still unsuccessful in providing commercial aluminum/ionic-liquid battery systems.
The low solubility of metal salts and metal oxides in ionic liquids and electrolytes containing ionic liquids poses a serious drawback for possible electrochemical applications of these solutions. The low solubility is assumed to be a consequence of the fact that most of ionic liquids contain weakly coordinating anions like tetrafluoroborate, hexafluorophosphate or bis(trifluoromethylsulfonyl) imide. Ionic liquids with fluorinated anions have lower melting points and viscosities than ionic liquids with coordinating anions like chloride or carboxylate ions, but their solvating abilities are very poor. The solubility of metal salts in ionic liquids can be increased by mixing the ionic liquid with coordinating additives with a low vapor pressure. An example is the addition of poly(ethylene glycol)s to ionic liquids. Other examples are the “deep eutectic solvents” such as a mixture of choline chloride and urea or a mixture of choline chloride and carboxylic acids. Additional approach to increase the solubility of metal salts in ionic liquids is to use ionic liquids with appending coordinating groups, so called “task specific” RTILs.
The latter ionic liquids are typically mixed with more conventional ionic liquids in order to lower their melting point and viscosity. An example for such an ionic liquid mixture is betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N]. This ionic liquid, bearing a carboxyl group, has a selective solubilizing ability for metal oxides. Among such oxides are uranium(VI) oxide, zinc(II) oxide, cadmium(II) oxide, mercury(II) oxide, nickel(II) oxide, copper(II) oxide, palladium(II) oxide, lead(II) oxide, and silver(I) oxide. Even though a variety of oxides were found to be soluble in this specific RTIL, iron, cobalt, aluminum and silicon oxides were found to be insoluble or very poorly soluble therein [Nockemann P. et al., J. Phys. Chem. B, 2006, 110, 20978-20992].
Metal deposition on various substrates is another field that presents challenges, particularly when the substrate is a self-passivated metal and also when the source of metal ions for deposition is an insoluble oxide or otherwise unreactive metal species. Some metal deposition processes still require the use of seeds, and such seeding make depositing result less uniform, especially in depositing on ultra-delicate structural features. For example, copper electroplating of the TaN/Ta diffusion barrier in Ultra Large-Scale Integration (ULSI) is a complex scientific and engineering problem which requires seeding and highly protected working environments, rendering scaling of the patterns for deposition, diffusion barrier layers manufacturing and electroplating itself a difficult task.
Additional background art includes U.S. Pat. Nos. 5,082,812, 5,411,583 and 5,587,142, and German Patent No. DE19731349.